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gypsum evaporite mineral, CaSO4.2H2O; sp. gr. 2.3; hardness 1.5–2; monoclinic; clear white, but sometimes shades of yellow, grey, red, and brown; white streak; vitreous lustre; crystals usually tabular, often with curved faces, but can occur massive or granular; cleavage perfect {010}, good {100}, {011}; occurs in bedded deposits in association with halite and anhydrite. It is very insoluble and therefore the first mineral to precipitate from evaporating sea water; usually succeeded by anhydrite and halite. Occasionally it results from the reaction of sulphuric acid on limestone in volcanic areas. It can also result from the secondary hydration of anhydrite. Selenite is the colourless, transparent form; satin spar is the fibrous variety; alabaster is the fine-grained variety and can be carved.

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Gypsum, a white mineral soft enough to be scratched with a fingernail, is hydrated calcium sulfate [Ca(SO4) 2H2O]. Gypsum often begins as calcium sulfate dissolved in an isolated body of salt water . As the water evaporates, the calcium sulfate becomes so concentrated that it can no longer remain in solution and crystallizes out (precipitates) as gypsum. Many large beds of gypsum have been formed in this way.

Gypsum occurs in a number of distinct forms, including a clear, parallelogram-shaped crystal (selenite); a white, amorphous form (alabaster, used for ornamental carving); and a fibrous, lustrous form (satin spar, used in jewelry). When ground up and heated to drive off its water, gypsum becomes a powder termed plaster of Paris. Plaster of Paris has the useful property of hardening in any desired shape when mixed with water, molded, and allowed to dry.

Gypsum is one of the most widely used minerals in the world. Some 90 countries mine gypsum, producing more than 100 million tons (91 million metric tons) annually. The construction industry has long been particularly gypsum intensive. In the late nineteenth and early twentieth centuries gypsum was widely used in plastering, which since the 1950s has been displaced by gypsum drywall (sheetrock). The average new U.S. home contains tons of gypsum drywall. Gypsum is also an ingredient in portland cement, which is used in the construction of bridges, buildings, highways, and the like, and millions of tons of gypsum are used annually as fertilizer. Small quantities of pure gypsum are essential in smelting, glassmaking, and other industries.

Low-grade gypsum is manufactured synthetically at coal-fired electric power plants as a by-product of pollution-control processes that remove sulfur from flue gas. Synthetic gypsum production exceeds 110 million tons (100 million metric tons) annually.

See also Mohs' scale

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gyp·sum / ˈjipsəm/ • n. a soft white or gray mineral consisting of hydrated calcium sulfate. It occurs chiefly in sedimentary deposits and is used to make plaster of Paris and fertilizers, and in the building industry. DERIVATIVES: gyp·sif·er·ous / jipˈsifərəs/ adj.

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gypsum (hydrated calcium sulphate, CaSO4.2H2O) Most common sulphate mineral. Huge beds of gypsum occur in sedimentary rocks, where it is associated with halite. It crystallizes in the monoclinic system. Varieties are alabaster, selenite (transparent and foliated) and satinspar (silky and fibrous). It is a source of plaster of Paris. Hardness 2; r.d. 2.3.

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gypsum hydrated calcium sulphate, from which plaster of Paris is made. XVII. — L. — Gr. gúpsos, of Sem. orig.
So gypseous XVII. f. late L. gypseus.

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gypsum (jip-sŭm) n. see plaster of Paris.

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